Abstract: In order to precisely evaluate the shielding effectiveness in the high-intensity electromagnetic pulse situation, a new time domain measuring method is presented. The shielding cavity with testing aperture is placed in the GTEM cell, the shielding material under test is installed on the testing aperture. The ouput voltage waveform is measured by monopolar antenna placed in middle position of the cavity. The time domain measured signal is applied by FFT, the shielding effectiveness can be obtained by the transformed frequency signal. The calculation shielding effectiveness is similar to the directively measuring result in frequency domain. The experiment results show that new measring method can reduced the influence of the local electric field enhancement to the shielding effectiveness measurement precision, is propitious to precisely evaluate the shielding effectiveness in the high-intensity electromagnetic pulse enviorment.